School/Research organisations

Abstract

17O solid-state NMR
spectroscopy is employed to investigate the cation disorder in
metal–organic frameworks containing two different types of metal
cations. Although NMR offers exquisite sensitivity to the local,
atomic-scale structure, making it an ideal tool for the characterisation
of disordered materials, the low natural abundance of 17O
(0.037%) necessitates expensive isotopic enrichment to acquire spectra
on a reasonable timescale. Using dry gel conversion and a novel steaming
method we show that cost-effective and atom-efficient enrichment of
MOFs is possible, and that high-resolution 17O
NMR spectra are sensitive both to the structural forms of the MOF and
the presence of guest molecules. For mixed-metal forms of MIL-53, NMR
can also provide information on the final composition of the materials
(notably different to that of the initial starting material) and the
preference for cation clustering/ordering within the MOFs. For Al, Ga
MIL-53, the distribution of cations results in a mixed-pore form upon
exposure to water, unlike the different structures seen for the
corresponding end members. This work shows that as good levels of
enrichment can be achieved at reasonable cost, 17O NMR spectroscopy should be an invaluable tool for the study of these important functional materials.